First panel added to a frame disappears

Question

Below is an example of adding two panels to a frame. Only one panel (the 2nd, red panel) appears.

Why does the first panel disappear?

import java.awt.*;
import javax.swing.*;
import javax.swing.border.EmptyBorder;

public class DisappearingPanelInFrame {

    DisappearingPanelInFrame() {
        JFrame f = new JFrame(this.getClass().getSimpleName());
        f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        f.add(new ColoredPanel(Color.GREEN));
        f.add(new ColoredPanel(Color.RED));

        f.pack();
        f.setVisible(true);
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                new DisappearingPanelInFrame();
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

class ColoredPanel extends JPanel {

    ColoredPanel(Color color) {
        setBackground(color);
        setBorder(new EmptyBorder(20, 150, 20, 150));
    }
}

Answer 1

• The default layout of a JFrame (or more specifically in this case, the content pane of the frame) is a BorderLayout.
• When adding a component to a BordeLayout with no constraint, the Swing API will put the component in the CENTER.
• A BorderLayout can contain exactly one component in each of the 5 layout constraints.
• When a second component is added to the same (in this case CENTER) constraint of a BorderLayout, this implementation of Java will display the last component added.

As to what would be a better approach depends on the specific needs of the user interface.